In his tribute, David's brother Mike Haines said the father-of-two was "brought up to know right from wrong".

He said: "David was like so very many of us, just another bloke. Born in 1970 to parents who loved us both, our childhood was centred around our family. Holidays in caravans and tents, days away as a family which we remember fondly.

"David and I were brought up to know right from wrong, although we might not with the innocence of youth have always chosen right. David was a good brother, there when I needed him and absent when I didn't. I hope that he felt the same way about me."

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Mike Haines went on to say that "in the right mood [his brother was] the life and soul of the party and on other times the most stubborn irritating pain in the ass."

"He would probably say the same about me," said Mr Haines.

Mike Haines said that after leaving school David Haines had worked for Royal Mail before joining the RAF as an aircraft engineer.

"He married his childhood sweetheart Louise and in the due process of time had a wee lass Bethany. He was - and no doubt wherever he is - exceptionally proud of Bethany."

"David served with the UN in the Balkans, helping people in real need. There are many accolades from people in that region that David helped. He helped whoever needed help, regardless of race, creed or religion. During this time David began to decide that humanitarian work was the field he wanted to work in."

ISIS had threatened to kill David Haines if world leaders did not bow to their demands.

Last night, the extremists posted a new video lasting 2minutes and 30 seconds which showed his beheading.

The Prime Minister rushed back to Downing Street from a society wedding in Hampshire and will chair an emergency meeting of COBRA on Sunday morning.

He condemned the murder of David Haines as "an act of pure evil".

The Prime Minister said: "This is a despicable and appalling murder of an innocent aid worker. It is an act of pure evil.

"My heart goes out to the family of David Haines who have shown extraordinary courage and fortitude throughout this ordeal.

"We will do everything in our power to hunt down these murderers and ensure they face justice, however long it takes."

He was joined by the the leader of the opposition Ed Miliband who said: "I am sickened at the disgusting, barbaric killing of David Haines."

President Barack Obama also condemned the murder and promised the United States would "destroy" ISIS.

(Image: Facebook)

President Obama said in a statement: "The United States strongly condemns the barbaric murder of UK citizen David Haines by the terrorist group ISIL (ISIS).

"Our hearts go out to the family of Mr. Haines and to the people of the United Kingdom.

"The United States stands shoulder to shoulder tonight with our close friend and ally in grief and resolve.

"We will work with the United Kingdom and a broad coalition of nations from the region and around the world to bring the perpetrators of this outrageous act to justice, and to degrade and destroy this threat to the people of our countries, the region and the world."

The new execution video begins with a news clip of David Cameron talking about Britain's policy towards "this appalling organisation".

This fades out before text appears which reads: "A message to the allies of America".

The video then shows David Haines, on his knees in an orange jumpsuit with his hands handcuffed behind his back.

An Islamic militant - believed to be the infamous Jihadi John- stands behind him holding a knife.

David speaks first, saying: "My name is David Cawthorne Haines. I would like to declare that I hold you David Cameron entirely responsible for my execution.

"You entered voluntarily into a coalition with the United States against the Islamic State just as your predecessor Tony Blair did following a trend amongst our British Prime Minsters who can't find the courage to say no to the Americans.'

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"Unfortunately it is we the British public who will pay the price for our parliament's selfish decisions."

The man believed to be Jihadi John then says: "This British man has to the pay the price for your promise Cameron to arm the Peshmerga against the Islamic State ironically he has spent a decade of his life serving under the same Royal Air Force that is responsible for delivering those arms.

"Your evil alliance with America which continues to strike the Muslims of Iraq and most recently bombed the haditha dam will only accelerate your destruction and playing the role of the obedient lapdog Cameron will only drag you and your people into another bloody and un-winable war.

The footage then shows what is believed to be the dead body of Mr Haines.

Another British hostage is then paraded to the camera.

Jihadi John says: "If you, Cameron, persist in fighting the Islamic State then you like your master Obama, will have the blood of your people on your hands."

Before tonight's video, ISIS militants had not responded to attempts made by David Haines' family to make contact.

Mr Haines was born in East Yorkshire but raised in Perthshire, and before his capture was living in Croatia with his second wife Dragana.

He has a 17-year-old daughter with his first wife Louise, and a four-year-old daughter with Dragana who described him as a "fantastic man and father".

Mr Haines was taken while working for ACTED, having previously helped people in Libya and South Sudan.

Militants from IS have beheaded two American journalists, Jim Foley and Sotloff, posting the evidence online in gruesome videos featuring a masked jihadist with a British accent.

Educated at Perth Academy secondary school, he has worked for aid agencies in some of the world's worst trouble spots, including Libya and South Sudan.

He was in Libya during its civil war in 2011, working as head of mission for Handicap International, which helps disabled people in poverty and conflict zones around the world.